1. Field of the Invention
The present invention relates in general to measuring of a noise factor and relates in particular to a noise factor measuring method and apparatus which permits simple, rapid and precise adjustment of phase differential.
2. Description of the Related Art
Presently, there are two known types of optical amplifiers: those based on optical fiber doped with a rare-earth element, such as erbium (Er); and those based on a semiconductor optical amplifier. A parameter for characterizing the performance of these optical amplifiers is a noise factor, and a method and an apparatus for determining the noise factor (NF) have been developed and disclosed in a Japanese Patent Application Number H6-17593.
In the case of the optical amplifier based on Er-doped optical fiber (shortened to optical fiber hereinbelow), when an excitation laser light is injected into the optical fiber, some atoms within the optical fiber are excited from a base level to a higher energy level. When the atoms return from the higher energy level the base level, light is emitted in the process. There are two modes of light emission: one is an induced emission process which occurs as a result of the atomic transition action induced by the excitation laser light; and another is a spontaneous emission process (referred to as SE process). Optical amplification is achieved through the induced emission process. The transition step (from high to low energy levels) in the induced emission process occurs rapidly, however, the transition step in the spontaneous emission occurs slowly relative to the speed of the induced emission process. In Er-doped optical fibers, the lifetime of spontaneous emission (i.e. lifetime of excited atoms) ranges from several milliseconds to several tens of milliseconds. The spontaneous light (referred to as SE light hereinbelow) generated in the spontaneous emission process is amplified within the Er-doped optical fiber, and is outputted as amplified spontaneous light (referred to as ASE light hereinbelow).
The noise factor (NF) of an optical amplifier is determined as follows. A subject optical amplifier whose NF is to be determined is irradiated with a pulsed laser light having sufficiently shorter pulsing cycle than the lifetime of spontaneous emission, and the resulting values of the electrical output power with and without the laser excitation are measured. That is, on the one hand, the emitted power PASE outputted during the time interval of spontaneous emission (without the laser excitation) is measured, and on the other hand, a sum of the laser output power PAMP and the emitted power PASE (i.e. PAMP+PASE) during the time interval of laser excitation is measured. Then, the noise factor is given by the following equation: EQU NF=(PASE/h..nu..A.BO)+1/A (1)
where h is the Planck constant; .nu. is the optical frequency of the laser light inputted into the subject optical amplifier; A is the gain of the subject optical amplifier; and BO is the transmission band width of the device for measuring the emitted power PASE of the ASE light.
The gain A of the optical amplifier can be approximated by the following equation: EQU A.perspectiveto.(PAMP-PASE)/PIN (2)
where PIN is a value of the input power of the laser light into the subject optical amplifier.
One problem in the method of NF determination presented above is that the pulsed laser light inputted into the subject optical amplifier must first propagate through the optical amplifier before it is outputted from the optical fiber, and therefore the laser output is delayed, where the delay time is dependent on the length of the optical fiber. It follows that subject optical amplifiers having differing lengths have different delay times. Additional delay is caused by optical fibers connecting the subject optical amplifier and an NF determining system. Therefore, in this approach to determining the NF of an optical amplifier demands that the various delay times be pre-determined, and that the phase relationship of the switched laser pulses be determined between the pulse phases measured during both time intervals with and without the pulsed laser excitation.